Circuit breaker



Nov. 3, 1953 Original Filed June 8, 1945 H. M. STROBEL 2,658,127

CIRCUIT BREAKER 2 Sheets-Sheet l IN V EN TOR.

50 W Mimi H. M. STROBEL CIRCUIT BREAKER Nov. 3, 1953 2 Sheets-Sheet 2 Original Filed June 8, 1945 1 In ww r I MHHHIdh n n nhl h l l l I I l INVENTOR.

Patented Nov, 3, 1953 umrs o PAT ENT orrl or:

2,658,127 ciitbfii'r BREAKER "How'ardM. Strobel, NtvYork, N. Y.

Original app ligation June Serial 598,269, new Patent No. 2,564,178, da'tedAugflst 14,4951. n via 'jt iand this application July It, 19'51,"Se"rial Noczsases interrupter 'structilre's wherein the ziiagnetieal ly E deflected arc eleinents "are moved along difienent types'o'f predetermined paths. This "patent application is a division'o'f the aforesaid patent application. I H

The primary object "of the invention is to provide improved apparatus'and methodforincrea'sing the number of subdivision'al are elements that may be formed in a dei'o ni z'atio'n circuit breaker where certain diinensionso'f'size are limited by practical space considerations, and to provide therein improved method and means for r'nagnetically deflecting are elements along 'pre determined paths. H a

In the conventional 'deioniza'tion circuit break er the main are, which is drawn out between the separable arcing contacts, is moved into an are receiving means comprisin spaced conducting plates adapted to subdivide the main are into a plurality or smaller primary arcs. The geometry of the spaced conducting mates-defines the term of path that the smaller primary arcs are to be moved along. The path so defined maybe apertended,fas when the final arc movement is translational, or recurrent, as when each primary arc is finally rotated like a cylindrical element at like a radial element along a substantially circul-ar path.

A more specific object of my invention is the provision of improved apparatus for a circuit interrupter having a plurality of deioniz'ation units racked side-hy-sid'e in series connection, each of said units having twofsucces'sive arrays each of which includes a plurality of s'paced'cor'F ducting arc splitting members, the first array of each unit being formed preferably of substan-" tially two spaced parallel planes plates with are receiving means arranged substantially transverse to the main arc path and having a suitable deflective means or magnetic circuit adapted to move the main are into its are receiving means, Q.

whereby the main arc is moved into the are receiving means of the first array of each deioniaation unit and separated into as many short or primary arcs as there are deionization units in series, and connecting means being provided be tween the first and second arflysof each di onliiiit so h'a-t e'aoh (if said primary arts in succ'e'ssively from 'one array to the 'oi'flier is eit n'fied and cheated to a position substantially askew to and "at right angles "to the main arc path, the "seconilarray of each deioniza'tion unit having arc receivin members arranged-sub stantia-lly transverse to the eiiten'de'd andiifiiitpr 'a-ry arcs and having provideda suitable defiecti-ve means or magnetic circiiitfadap'ted to me e each r the ext'ejna'edahd oriented ptim'ett arcs into its test tive fare receiving meats, whereby eachexteniied ana-ttientea priniary arc move into thearc receiving 'ineansof'the secand "array 01" deionization unit and further separated into -'a plurality" of sh'orterfor secondary arcs which are 'the 'nfin ade to mete-siting a recurr'ent path as'die'fined by the "are running 'in'embers of each :leioni'zfation unit of the improved apparatus iinti-l "extinguished.

More particularly, their'ivention consists in the system "and method hereinafter described, illustrated in the accompanying drawings and defined in 'thfe claiin's hereto appended, it being nnclerstood that 'tar ious changes in form, arrangement and details both of circuits and of "method within the "scope of the claims ma be resorted to without departing from the spirit or sacrificing anyefthe advantagesof the invention.

A clearer understanding of the operation "er the invention and its "improvement over known methods can be obtained byfrefet bite to the fol lowing figures and the descriptions relating "thereto. y Fig. 1 "is a pictorial view of one section-er Unit of the improved 'deion'ization device Whose structure is adapted to define the paths "of the are elements to give final radial rotation in a recur rent path.

Fig. 2 shows two or the denductive cyIind'ri calpi'e'ces used in the dvijceof Fig. 1.

Fig. 3 shows three; of the insulative pieces which are used in supporting the conductive eylindric'al pieces of Fig. 1. H

Fig. 4 shov'is-pictorialIy the new or current through a circii'it breaker sing a-pli'i'rality of the units shown iii Fig. 1. Fig 5 haws ajvie'w bartiy seatren anti partly n elevat'ionjof t'h'e are extinguisher of my invention which uses diohiza'tion 'iinits substantially similar to the one in Fig. 1'; the section has been taken alone the line v 'v in the plan viw'orr'iac. Fig. 6 sho ws a plan view-of an arc extinguisher With six of the improved deionization units (which are substantially similar to the one in Fig. 1) assembled in series.

Fig. 7 is a cross sectional view through the arc extinguisher of Fig. taken along the line VIP-VII.

Fig. 8 is a cross sectional view through the arc extinguisher of Fig. 5 taken along the line VIIIVIII.

Fig. 9 is a cross sectional view through the deionization unit 300 of Fig. 5 taken along the line IXIX.

Fig. 1 shows a structure for defining the paths and motions of the are elements to give final radial rotation thereof in a plurality of concentrio circular recurrent paths. The primary arc splitting plates are indicated by 3| and 32. The inner cylindrical metallic member 33 is joined to the upper end of metallic plate 3|, and the outer cylindrical metallic member 34 is joined to the upper end of metallic plate 32. The insulating members 29, 35 and 35 are adapted to support the cylindrical secondary arc splitting plates 31a, and 3119. For clarity, only two cylindrical plates 31 are shown here, although it is obvious that as many such cylindrical plates 3'! can be used as considerations of spacing and size permit. (In Figs. 5 to 9, for example, five concentric cylindrical secondary plates 31a to 316 have been used.) Piece [9 is an insulative baffle. The numbers 38 and 39 indicate the edges of the primary arc splitting plates 3! and 32 (or 34) which serve as arcing guides to extend and orient the primary arc existing between the points 43 and M.

Referring to Fig. 1, it is noted that the cylinders which serve as the secondary arc splitting plates are formed by cutting out a 180 sector from the cylinders as shown. This would require that the extended and oriented primary arc element should be in a vertical radial position as it entered the secondary arc splitting plates. However, as a matter of practical design, it might be more desirable that the said primary arc element should be in a horizontal radial position and be moving upwards as it entered the secondary arc splitting plates. (The units 39 shown in Figs. 5 to 9 are made in this manner.) The cutting out of the 180 sector from the cylinders as shown here in Fig, 1 served to clarify the drawings and the description of the operation of the device. However, by cutting out a 90 sector from all of the cylinders, as indicated by the dotted line extension of the cylinders 31d and 31b in Fig. 2 and as shown in Figs. 5 to 9, the primary arc element would be in a horizontal position while entering the secondary are splitting plates. With such a construction, of course, it would be necessary to modify pieces 35 and 35 as to their shape and the locations of the venting holes as is shown in Figs. 7 and 8. These minor mechanical details are more or less obvious. Also, the are receiving means of 3'! may be notched similar to 3117 of Fig. 2, or similar to 3| of Fig. 1.

Fig. 2 shows the conductive cylindrical secondary arc splitting plates 37a, 31b. Fig. 3 shows the three insulative members 35, 29 and 35 in the same relative space positions that they occupy in the assembled unit 30 shown in Fig. 1, These members are provided with grooves to retain and support the cylindrical members 33, 34, 37a and 31b.

Figs. 5 to 9 show elevation, plan and cross sectional views of an arc extinguisher using six deionization units 300. to 30]. Each of these deionization units is similar to the one shown in perspective in Fig. 1, except that five cylindrical secondary arc-splitting plates 31a to 31s are shown in lieu of the two shown for descriptive purposes in Figs. 1 and 2. The identifying numbers used to indicate the parts of unit 38 in Fig. 1 are also used to indicate the same parts in Figs. 5 to 9. Referring to Figs. 5 to 8, the uprights 2 la, 2 lb are made of insulating material and support the cross pieces 20a and 2017, also made of insulating material, to form a box or rack to hold the six deionization units 30a to 301. The arcing horns 2'1 and 28 are bolted to the uprights Zlb and 2Ia respectively, as shown. The stationary contact 24 is supported from the arc horn 2l' but insulated therefrom by the block of insulation 8. The moving contact 25 may be actuated to open position by any suitable type of operating mechanism which may be provided with main contacts for normally completing the circuit through the circuit breaker. The line terminal 22 of the circuit connects to the stationary contact 24 and the other line terminal 23 to the arc horn 28 and the moving contact 25. The coils em and Bib of Figs. 5, 6 and '7 are positioned in grooves provided in the insulating cross pieces 23a and 201). When energized, the coils fila and fill) provide a magnetic field transverse to the path of the main arc and thus deflect the main are into and past the primary arc splitting plates 3| and 32 of each deionization unit 38. The coils Bid and Bi?) are connected in series by the jumper 6011 (Fig. 7), with one end of the coil 6 lb connected to the stationary contact 24 through the conductor 50 (Figs. 5 and 7), and the other end of coil 51a returned to are horn 2'! through the conductor 501) (Fig. 7). The coils 64a and 64b of Figs. 5 and 6 are positioned in grooves provided in the insulating uprights 2m and 2H). When energized, the coils 64a and 64b create a magnetic field through the secondary array of concentric cylinders of such directions as to be parallel to the common axis of the cylinders and thus transverse to the paths of the secondary radial are elements formed between the cylindrical secondary arc splitting plates 3] in each deionization unit 30. The coils 64a and 64b are connected in series by the jumper 63a (Fig. 5), with the other end of coil 64a connected through conductor 63 to the end secondary are switching plate 31a of deionization unit 30a (Fig. 5), and the other end of coil 54b returned to line terminal 23 through the conductor 63b (Fig. 5).

Referring to Fig. 5, when the arcing contacts 24, 25 are separated an arc is formed between them and the current flow is line terminal 22 to stationary contact 24, to movable contact 25, and return to line terminal 23. The are flame is then moved upward between the arc horns 27 and 28, the transfer of one end of the are from stationary contact 24 to are horn 2T automatically switching in coil 6| so that the line current must now flow from line terminal 22, to stationary contact 24, through conductor 33, coil El to are horn 2'1, and thence through the arc to are horn 23 and return to the line terminal 23. The connections and current flow through the coil 5! are such as to create a horizontal magnetic field transverse to the main arc discharge. The arrow 52 in Figs. 1 and 4 indicates the direction of the magnetic field H produced by the coil 6|. Referring back to Fig. 5, the magnetic field defiects the arc flame upwards into the primary arc splitting plates 3i, 32 of each of the six deionization units 30a to 30f shown. The main arc is thus divided into a plurality of shorter arcs or primary arcs by the six deionization units t'fla to 3 In this regard, it is to be pointed out that the apparatus and method used to createand deflect the mainarc into the primary arc splitting plates, where it is divided into a plurality of primary arcs, follow conventional practice and are well known to the art. According to this invention (the deionization unit 30 of Fig. 1), the primary arc existing between each pair of primary are splitting plates iii, 32 or each deionization unit 313 (Fig. 1 or '5) is then moved upwards, extended and oriented to a line-of position above andsubstantially at right angles to the line of position of the original primary arc. E'ach extended and reoriented primary arc is then moved into the are receiving means of the secondaryarray of its deionization unit 30 where it is further 'su-bdivided into a plurality "of secondary arc elements. These secondary are elements are then rotated radially by magnetic means in a -recurrent circular path as defined by theconcentric cylinders of the secondary array until the arc is extinguished.

By referring to the perspective view of one of the deionization units .30 shown in Fig. 1 it will become clear how the structure of a deionization unit 30 extends, orients, and further 'subdivides each primary arc element into a plurality of secondary are elements and then causes them to rotate radially along a circular recurrent path. As mentioned above, when the main arc strikes the primary arc splitting plates 3| and 32 it is subdivided to form a primary arc element between plates 3| and 32. The primary arc element moves upward between these plates until the hot spot '40 on plate 34 reaches the arcing runnertt, and the hot spot! on plate 32 reaches the arcing runner 39. Since the primary are between tlfi and it is still being deflected upwards by the magnetic field, the hotspot 40 movesalong 38 to point 40a, while hot spot 4| moves along 39 to point Me. It will be noted that this movement of the are hot spots along the arcing runners results in both extending the primary arc and in substantially reorienting it to a position substantially at ri ht angles to the direction of the main arc. Ultimately the hot spot 40 reaches the inner cylindrical member 33 of plate 3|, and hot spot 4! reaches the outer cylindrical member 34 of plate 32. In this position, it will be noted that the primary am has been extended between the cylinders 33 and .34, and reoriented to a radial position substantially at right angles to the main arc. As the extended and oriented primary arc is further deflected by magnetic or mechanical means, it will strike the are receiving edges of the secondary arc splitting plates 31a and 3'52), which then subdivide the primary .arc into a plurality of radial secondary are elements. It is to be noted that the secondary are elements are radial to the axis of the cylinders, so that when a magnetic field B5 is applied parallel to the axis of the cylinders the radial secondary are elements are rotated counterclockwise between the surfaces of the cylindrical members. The insulative end piece 35 (see Fig. 3) has its inside surface cut as a spiral wedge, which serves to guide the radial secondary are elements along the axis into the cylindrical region of unit 30 between the disc like insulative members 29 and 38. In this region the secondary are elements can be radially rotated by the magnetic field 65 around the circular recurrent path defined by the concentric cylinders until extinguished. .As shown in Fig. 3, the end disc 36 can be provide'd with slots to aid inventing the air heated by the arcs.

A plurality of the units -30 shown in Fig. -1 can be racked side by side in a supporting box like structure 20- 21, Figs. '5 to 7. When properly racked the axis-of each unit 30 will line up along a common line, and each insulative end member 36 and metal plate 32 will be pressed against the plate 31 of the adjacent unit '30. 'Whenso-racked, if we consider two units -30a and 301), with unit 30a in front of unit 302), it will beinoted that the piece 35(1)) of the back unit will be next to the piece 36-111) of the front unit so as to cover up the left hand side-o'fpiece 3'fi (a For this reason, no vent holes are shown in the left hand side of the insu'lative piece "36. However, it will be observed that the insulative piece '35 can be cut thinner at its lower half so as to provide space for additional ventin'g holes in piece 36 if required.

It'is to be noted that the cylindrical plates 3'1 and the end plates 33, 3 4 which make up the secondary array-of arc splitting plates in Figs. '1 and 5to Qconstitute a-closed loop. Furthermore, if the deionization units 3i! are used to interrupt alternating currents the coils which supply the magnetic deflecting fields must also be energized with alternatingcurrents. As a result, the varying magnetic fields will induce eddy currents in the closedlo'opsof the cylindrical p1ates3'3,=3-l and 37 by transformer action. These eddy currents may be prevented by the conventional method of cutting a longitudinal slot in each cylindrical member which forms a closed loop, taking care to stagger the position of each slot with respect to the position of the slots in adjacent cylinders. It will be obvious, of course, that when the deionization units 3 0 are used to interrupt direct current circuits this problem will not arise, and. the slots may be omitted.

Fig. 4 is .a schematic sketch of the flow of current through a-deionization circuit breaker utilizing a plurality of units 30 (36a, 30b, 380, etc.). rhe coils (-6! and 64 in Figs. 5, 6 and 7.) for producing the indicated magnetic fields H, arrows 62 and 65, are not shown here. The numbering of the parts in Fig. 4 corresponds substantially to those used in Figs. 1 2, and 5 to 9. The diagram shows the flow of current through the imits 351 after the main arc has been finally subdivided into secondary are elements which are being radially rotated in a circular path defined by the concentric cylindrical surfaces of the device. At this time, the cur-rent (instantaneous) flow is, as indicated by the current arrows, along 5!), Si, 52 to 33, 53 to 3m, 3122, 34, along '54 to 32, and so on through the succeeding units 3 0..

The above describes the basic operation of my improved deionization circuit breaker as applied to the structure shown. In the interest of clarity in the drawings the members or" the various structures .and their basic operative positions have been depicted as simply :as possible, in order to better illustrate the essential character of the invention. As a matter of practical construction, it is obvious that the insulative members supporting arc splitting members can be slotted and grooved to receive the edges of the arc splitting members, and thus assist in preventing the arc flame from flashing around the arc splitting members. In order to assist in guiding the moving are elements along the are running paths in the various structures as are defined by the conducting arc splitting members and the sides of their insulative supporting members, the

aerate? surface of the conducting members can be given a central raised contour along the line of the defined are running path. In the drawings the conducting members are shown with square edges, but it is obvious that these could be formed with their edges rounded in such a manner as to reduce the concentration of voltage stresses. Similarly, mechanical changes can be made in notching the are receiving means of the plates, in making provisions for venting, and for preventing creepage and voltage breakdown in accordance with principles commonly employed in the art, without departing from the spirit of the invention as shown and described.

In the drawings shown the various structures have been adapted to extend and orient the primary arc element to a line of position substan tially at right angles to the line of the main are before dividing each primary are into secondary are elements. It is obvious that the 90 angle of orientation of the primary arc element as shown is not a necessary restriction. For example, in Figs. 1 and 5 the secondary array is shown oriented at a 90 angle with the line of the primary array, but as far as electrical considerations are concerned, could just as well be at some other angle, such as 45, for example. As a matter of practical construction, however, the structure shown with the 90 angle of orientation is simpler than that which would be required by some other angle of orientation.

In the illustrations given the arc is deiiected into arc splitting members arranged in two successlve arrays, which are designated as a primary array and a secondary array. Two arrays substantially as described give a relatively simple physical construction of the device, and permit ready extension and orientation of the are to a line of position in space where an array comprising a plurality of secondary are splitting members can be conveniently formed. However, it is readily apparent that are splitting members could be introduced at intermediate positions between the original position of the primary are and the final position of the successively extended and oriented arc. Such an increase in the degree of arrays would follow readily from the ideas disclosed in this invention, but from a practical viewpoint it is apparent that such an operational cascading of arrays would result in a more complex physical structure.

The use of this invention, in which a portion or element of the arc is extended and oriented to a position which permits the use of increased number of arc splitting members, gives rise to many advantages. The are splitting members can be constructed from sheets of conducting material formed into flat or cylindrical shapes. That is, the curved cylindrical surfaces required are deveiopable surfaces which can be cut out or developed from flat sheet metal stock and then rolled or curved into the desired cylindrical surfaces. The use of an increased number of arc splitting members for a given consideration of size and space enables the device to interrupt greater voltages and currents than would otherwise be possible. The device of Fig. 1 provides for a recurrent circular are running path, in which the secondary are elements are rotated radially. It will be observed that a simple transverse magnetic field parallel to the axis of the cylinders is all that is required to give the desired recurrent radial rotation of all the final secondary are elements. Because of the cylindrical construction of the secondary array of the deionization units 3B, Figs. 1 and 5, the coil turns required to produce this longitudinal magnetic field (65 in Fig. 1) could very conveniently be wound around the cylinder 34 of each unit 30 so that when they are racked as shown in Fig. 5 a uniform longitudinal magnetic field would result.

I claim:

1. In a circuit interrupter, means for drawing a main are between a pair of separable contact members, means for dividing said main are into a plurality of series related primary are elements, means for extending and orienting at least one of said primary are elements to a position substantially at right angles to the main arc path, means for dividing at least one of said oriented primary are elements into a plurality of series related secondary are elements, and means for radially rotating at least one of said secondary are elements along a closed recurrent are running pathbetween spaced conducting members around an axis substantially parallel to that of the main arc path until extinguished.

2. In a circuit interrupter, a primary series array of parallel spaced conducting members, a secondary array of spaced conducting members for substantially each pair of members in the primary series array, said primary series array being positioned along a line substantially parallel to and above that of the main arc path and having the plane of the are receiving means of its conducting members in a plane substantially transverse to the main arc path, said members of substantially each of said secondary arrays comprising a plurality of substantially concentric conducting spaced cylinders, the common axis 01' said concentric cylinders being substantially parallel to the main arc path, substantially each of said cylinders having a longitudinal offset portion adapted to receive a primary are element and to subdivide it into series related secondary are elements, means for drawing a main are between a pair of separable contact members, means for causing said main arc to pass into the are receiving means of the primary array where it is subdivided into a plurality of series related primary are elements, means for causing substantially each primary arc element to be extended and oriented so as to pass into the are receiving means of a secondary array where it is subdivided into a plurality of series related radial secondary arc elements, and means for causing substantially each radial secondary arc element to move along a substantially circular are running path as defined by its pair of spaced conducting concentric cylinders in the secondary array until extinguished.

3..In a circuit interrupter, a primary series array of parallel spaced conducting primary members, a secondary array of concentric spaced conducting secondary members for substantially each pair of primary members, said primary members including substantially plane flat plates with a lower portion thereof adapted to receive and series divide an arc, the plane of the are receiving portion of said primary members being arranged substantially transverse to and above the main arc path, substantially each pair of primary members having an extension for each primary member adapted to form a, pair of spaced and substantially concentric end plate secondary members in said secondary array, each of said end plates including substantially a cylindrical surface with a portion thereof adapted to receive the terminal end of an arc, the plane of the are receiving portion of said end plate members being arranged substantially vertical and at an angle to the plane of the are receiving portion of its primary member, the adjacent surfaces of said cylindrical end plates being spaced substantially parallel to each other, the spacing between said cylindrical end plates being greater than the spacing between their respective primary plates, a plurality of spaced conducting secondary members arranged concentrically between each pair of said cylindrical end plate members and having their active surfaces substantially parallel to those of said end plate members, said secondary members comprising substantially concentric cylindrical plates with a longitudinal ofiset portion thereof adapted to receive and series divide an are, means for drawing a main are between a pair of contact members, means for causing said main arc to pass into the are receiving means of the primary array where it is subdivided into a plurality of series related primary are elements, means for causing substantially each primary are element to be extended and oriented so as to pass into the are receiving means of a secondary array where it is subdivided into a plurality of series related radial secondary are elements, and means for causing substantially each secondary arc element to move as a radial element along a substantially circular recurrent are running path as defined by the active surfaces of its pair of spaced concentric cylindrical members in the secondary array until extinguished.

4. In an arc extinguisher, a pair of parallel substantially plane primary members and a plurality of substantially concentric cylindrical secondary members, said members being formed of conducting material, said conducting members being spaced and supported by insulative members with the plane of the primary members being substantially transverse to the common axis of the concentric cylindrical secondary members, said insulative members defining an endless path between substantially each pair of secondary members, defining an approach path extending from adjacent the point where the arc is established between the primary members to the are receiving means of the secondary members, and defining a helicoidal path extending from the point where an arc is established between a pair of secondary members into said endless path, and means for setting up a magnetic field for moving the are along said approach and helicoidal paths into said endless path and for rotating the arc to move it along said endless path.

5. In an arc extinguisher, a pair of parallel substantially plane conducting primary members and a plurality of substantially concentric cylindrical conducting secondary members, the inner secondary member being conductively joined to one primary member, and the outer secondary member being conductively joined to the other of said primary members, substantially each intermediate secondary member having an offset portion and a notch to form an alined groove in which the arc may play, said primary members having an arc approach path adapted to extend and orient the arc into a position adjacent to the are receiving means of the secondary members, substantially each pair of a plurality of said secondary members having a recurrent arc path extending therearound at one side of said groove, means for moving the are into the arc receiving means of said primary members, means for moving the are along said are approach path, and means for establishing a magnetic field longitudinally through said cylindrical conducting secondary members and across said groove for moving the are into said cylindrical conducting secondary members and splitting it up into short series related arcs and for rotatin said short arcs about said recurrent paths.

6. In a circuit interrupter, a pair of spaced pole areas defining a field in a magnetic circuit, a pair of stationary parallel spaced conducting primary members substantially parallel to said pole areas and having their upper extensions positioned in space between said pole areas, a plurality of concentric cylindrical conducting secondary members positioned longitudinally in said field and in the space between said upper extensions of said primary members, a plurality of said cylindrical secondary members having a helicoidal are receiving portion, said primary members having an arc approach path adapted to extend and orient the are into a position adjacent to the are receiving means of the secondary members, means for initially establishing the arc in space adjacent to the are receiving means of said primary members and transverse thereto, and means for setting up a magnetic field between said pole areas and across said are establishing means for moving said are from said means and establishing it first between said primary members and then in said approach path and finally among said cylindrical secondary members.

7. In an arc extinguisher, a primary and a secondary group of spaced conducting plates, said secondary group including sets of substantially cylindrical and concentric plates, a plurality of said plates in said secondary group being electrically insulated from the plates in the primary group and being positioned substantially in sets between pairs of primary plates with their surfaces substantially perpendicular to the plane of the surfaces of the plates in the primary group, substantially each end plate of a secondary set being connected to but one primary plate, and means for causing the are to play with a part thereof first in the primary and then in the secondary group of said plates.

HOWARD M. STROBEL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,080,611 Johnson May 18, 1937 2,109,226 Austin Feb. 22, 1938 

